Translucent polycrystalline alumina (PCA) ceramic has made possible present-day high-pressure sodium (HPS) and ceramic metal halide lamps. The arc discharge vessels in these applications must be capable of withstanding the high temperatures and pressures generated in an operating lamp as well be resistant to chemical attack by the fill materials.
In HPS lamps, the discharge vessels are tubular as shown in FIG. 2, whereas for ceramic metal halide lamps discharge vessels can range from a cylindrical shape to an approximately spherical shape (bulgy). Examples of these types of arc discharge vessels are given in European Patent Application No. 0 587 238 A1 and U.S. Pat. No. 5,936,351, respectively. A bulgy-shaped discharge vessel is shown in FIG. 1. The bulgy shape with its hemispherical ends yields a more uniform temperature distribution, resulting in reduced corrosion of the PCA by the lamp fills.
In the past, some of the key elements in sintering polycrystalline alumina (PCA) to translucency involved the use of (1) a high-purity powder, (2) a small concentration of a MgO sintering aid, and (3) sintering in an H2-containing atmosphere. It has been reported in the literature that air, N2, He, and Ar atmospheres may not be used, but H2, O2, or vacuum did permit the attainment of translucency. This was due to the solubility of the gases in the lattice and grain boundaries allowing entrapped gaseous species to diffuse to the surface. In a vacuum environment, or in a gaseous atmosphere that is soluble and diffused rapidly in PCA, the sintering process is not kinetically limited, and pore-free microstructures are achieved. Later work has indicated that translucent alumina may be sintered in dissociated ammonia (25% N2-75% H2) and even in a CO atmosphere. The sintering of alumina has been reported in N2—H2 atmospheres containing as low as 2% hydrogen. Because of cost and safety issues, it would be desirable to eliminate the need to add hydrogen gas and use nitrogen gas only. However, a N2 atmosphere alone has not been able to produce translucent PCA